1. Field of the Invention
The field of the invention is operational amplifiers and particularly relates to operational amplifiers for driving fixed load impedances.
2. Prior Art
Many operational amplifiers in the prior art provide voltage amplification. Most operational amplifiers have two or more stages of gain. Multi-stage operational amplifiers have a tendency to phase shift their output signal with respect to their input signal. In order to maintain stability in the amplifier, the phase shift of the output signal must be less than 180 degrees at unity gain frequency. In order to satisfy this condition, additional phase compensation circuitry is used. However, phase compensation circuitry severely reduces the bandwidth of the amplifier. Also the phase compensation circuitry introduces a delay in the processing of the output signal. Thus, having phase compensation circuitry in an operational amplifier reduces both the bandwidth of the amplifier as well as the amplifier's speed.
An example of an operational amplifier having ultra-low input current and low power dissipation is the Intersil 76XX series low power CMOS operational amplifier. A particular circuit configuration used in those amplifiers is described in U.S. Pat. No. 4,529,948 (Bingham). In these circuits the second stage has a gain of 30-40 dB, which means that for high impedance loads, the output of the second stage does not match the load. These circuits are load dependent, and phase compensating circuits are required for their operation. Also, to achieve unit gain these devices require the use of a capacitor. Therefore, these prior art operational amplifiers suffer from reduced bandwidth and speed.
It is an object of the present invention to provide a high speed amplifier that matches the output signal to the output load. Another object of the present invention is to provide a high speed amplifier that drives a predetermined load over a large bandwidth. An additional object of the present invention is to provide an amplifier having a second stage of unity gain. Yet another object of the present invention is to provide an amplifier having a minimal phase shift without having compensation circuitry. A further object of the present invention is to provide an amplifier that can provide unity gain stability without the use of capacitors.